The prevalence and degree of obesity has increased dramatically over the past 2 decades. Obese children have a high probability of becoming obese adults. For example, a child who remains obese through adolescence has ~ 28 to 1 chance of attaining normal weight as an adult. Sedentary behaviors and nutritional preferences have their origins in childhood and are strongly associated with obesity. Establishing enthusiasm for physical activity (PA) and consumption of nutritious lower energy and fat snacks may act to promote and preserve interest in these healthy behaviors, as children grow older. Children are more adaptable to changes in lifestyle than adults, and prevention is more feasible than treatment;therefore, we believe children are the ideal recipients of our proposed intervention. We have completed a pilot project "Prevention of obesity in YMCA After School Programs," (DK63458) that successfully increased PA and diminished energy and fat in snacks. We now propose a full scale, cluster randomized, controlled trial of this intervention in 34 YMCA after school programs. We will train the YMCA staff to deliver at least 225 min of moderately vigorous PA and we will alter the snacks to diminish energy and fat. Our primary aim is to decrease gains in BMI in intervention children compared to standard care control. Our secondary aim is to show improvements in chronic disease risk factors in intervention children compared to control including fitness, body composition, blood chemistry (glucose, insulin, lipids), and blood pressure. Our third aim is to provide a rigorous process analysis to document the fidelity and extent of the intervention and to identify factors such as characteristics of the staff, students, and parents, training and support, physical environment, etc. that may be associated with successful intervention implementation. The YMCA is the largest provider of after school programs in the US with over 8,000 sites. If successful, we have tremendous potential for dissemination of the intervention through the national YMCA network.